The invention relates to a detection device for metal melts, in particular molten iron. The device has a sample cup and a blob containing a carbide stabilizer and a hydrogen releasing material is arranged in the sample cup.
During the processing and preparation of molten metal, particularly molten iron, it is desirous to monitor certain chemical constituents of the metal. One common means for doing so is the use of disposable phase change detection devices, which measure the temperature of a sample of the molten cast iron during solidification in order to detect the temperatures of the phase changes. Such phase change detection devices typically comprise a mold body having a cup-like shape and an upper open end for receiving a sample of molten metal. The devices also typically include a thermocouple extending into the cup below the surface of the as-poured molten metal sample. One such conventional phase change detection device is described in U.S. Pat. No. 3,267,732.
Typically, an operator scoops a sample of molten metal from a batch of the molten metal using a small spoon or ladle, and then pours the sample of molten metal into the sample mold of a detection device. The thermocouple continuously records the temperature of the metal as it solidifies. From the phase change temperature measurements of the solidifying metal sample, certain properties and aspects of the chemical composition of the cast iron sample, such as, but not limited to, carbon content, silicon content, and degree of saturation or carbon equivalent level, may be predicted. The operator can then utilize this information to make any necessary adjustments to the molten metal bath before casting.
In certain circumstances, such as for hypercutectic irons, it is useful to achieve a graphite-free, white solidification of the sampled iron. The term “white solidification” is a common term in the art and refers to an as-cast structure dominated by the solidification of iron in a carbide phase that, when fractured, appears “white.”
U.S. Pat. No. 3,546,921 (“the '921 patent”) teaches that the addition to the molten iron of a pellet comprising a carbide stabilizing element or compounds of such elements will promote the white solidification. However, the '921 patent does not achieve optimal white solidification because the pellet tends to rise to the surface of the molten metal as a slag, or tends to burn in atmospheric oxidation, such that it is not available for white solidification. Further, if the molten iron has a fairly high carbon content or if the molten metal has been heavily inoculated, the likelihood of achieving a total white solidification based on the pellet addition of the '921 patent is low.
U.S. Pat. No. 4,003,425 discloses that coating the inside of the sample mold with a material containing water to be liberated at the temperature of the molten iron, will improve the effectiveness of the aforementioned carbide promoting additives. In this instance, water is a vehicle whose purpose was to make available hydrogen to alloy with the metal. The improvement of an iron, especially hypereutectic irons, to solidify in a white structure is promoted by the presence of hydrogen.
U.S. Pat. No. 4,029,140 (“the '140 patent”) adopts this type of coating for use in promoting the carbide reaction in a disposable sample cup. The coating contains a carbide stabilizing element or compound and a material containing loosely combined water or some hydroxyl groups. The water or hydroxyl groups are retained after drying the coating, but are freely liberated from the coating at the temperature of the molten metal.
However, the method of applying the coating of the '140 patent to such conventional disposable phase change devices was limited in its usefulness, because both the walls of the sample cup and the thermocouple itself were coated. As a result, these conventional devices suffered from a thermal lag in the thermocouple. U.S. Pat. No. 4,274,284 purports to eliminate the thermal lag by the addition of an ablative coating which ensures that the thermocouple junction is exposed to the molten metal when the sample is poured into the device.
However, the multiple coatings negated the purported economical benefit of the '140 patent. In addition, the above-described coatings were applied were to be thin, which was found to be a major drawback. Specifically, it was found that the conventional coated sample cups cannot be completely filled with molten iron due to the violent release of hydrogen from the thin coating and due to the carbide promoting material rapidly boiling away from the thin coating, rather than alloying with the metal. As such, the volume of metal remaining in the sample cup was insufficient for obtaining temperature measurements. In turn, the amount of carbide stabilizing additives to be added to the molten metal in order to effectively promote white solidification could not be reliably predicted.
Further, the carbide promoting materials and the hydrogen releasing substances of the above-described prior art coatings have melting points below that of iron and boiling points near the temperature of the phase changes that are to be monitored. Thus, even with extreme care, a reaction of the coating materials with the molten metal is to be expected, and the extent of this reaction is of importance providing a controlled alloying of the additives.
Instead of a paint or coating, U.S. Pat. No. 4,059,996 discloses a blob of material which is fixed to the bottom of the sample cup. The blob of material includes a carbide formation promoting material, a refractory material and a material for evolving hydrogen (i.e., water glass) upon contact with the molten metal. The refractory material aids in preventing the carbide formation promoting material from being burned up quickly and mixing too quickly with the molten metal. The blob is initially in the form of a fluid mixture that is deposited in the sample cup, and is then dried to a solid in an oven.
U.S. Pat. No. 4,515,485 (“the '485 patent”) also discloses the use of a blob of material. However, the blob is disposed in a recess of a bottom wall of the sample cup, so that the surface area of the blob exposed to the molten metal is limited, and thus the amount of hydrated material exposed to the molten metal is limited.
None of the above-described prior art coatings and blobs satisfactorily achieves optimal white solidification for all compositions of casting irons. The reason for this failure is that each of the above-discussed prior art devices and methods fails to recognize and appreciate the problem of the environmental instability of the materials of the coatings and blobs. Specifically, the present inventors have found that the materials utilized in the above-discussed prior art coatings and blobs will, over time, lose moisture to or absorb moisture from the surrounding environment, during storage and transport to the location of use and also while awaiting use after delivery to the location of use.
For example, the prior art detection devices are provided with the coating or blob at the time of manufacturing of the device, well in advance of the time when these devices will actually be used. The manufactured devices are then boxed, palletized, shrink-wrapped and transported by land, sea or air to be unwrapped and used in another environment or location. However, during this time, the detection devices are typically subjected to uncontrolled transport and storage environments. In addition, the location at which the detection device is ultimately unwrapped and used may also be under conditions of extremes of temperature and humidity.
The present inventors have thus found that the prior art detection devices, and particularly the hydrogen releasing capacity of the coatings and blobs of these devices, are unstable because the coating or blob materials are susceptible to changes in their hydration levels. Specifically, the coatings or blobs are susceptible to taking on additional hydration in a moist environment and losing hydration in a sufficiently dry environment.
Although loss of moisture over time is problematic, the present inventors have found that exposure to damp conditions is essentially detrimental for the prior art coatings and blobs. Specifically, in damp conditions, where the prior art coatings and blobs are susceptible to uncontrolled moisture absorption from the surrounding damp environment, uncontrolled boiling of the molten metal sample results. Accordingly, as described above, the volume of metal remaining in the sample cup is insufficient for obtaining temperature measurements and, in turn, the predictability of the carbide stabilizing additives of the prior art coatings and blobs is negatively impacted.
The '485 patent recognizes that uncontrolled boiling leads to changes in the amount of molten metal remaining in the sample cup during solidification, which thus produces different results when the blobs are of a uniform size. However, this prior art device does not satisfactorily eliminate or reduce boiling. Thus, the device of the '485 patent fails to recognize the problem newly discovered by the present inventors, namely that the occurrence of uncontrolled boiling is actually the result of absorbed moisture in the carbide promoting materials. Indeed, the present inventors found that some environmentally absorbed water inevitably accumulates on even the limited exposed surface area of the blob of the '485 patent. As a result, excess boiling still occurs, thereby failing to yield the desired improvement.
Thus, the above-discussed prior art devices and methods all fail to recognize the existence of dampness and fail to address how to prevent damp conditions which occur as a result of the environmental exposure of the coatings or blobs.